This invention relates to a method of creating an electroluminescent device and to a device created using the method.
The integration of optoelectronic and electronic components from different origins and substrates makes possible many advanced systems in diverse applications in photonics. To this end various hybrid integration methods and technologies have been developed.
Examples of heterogeneous integration are flip-chip bonding, epitaxial lift-off, substrate removal and applique bonding, and micro-robotic pick and place.
Heterogeneous integration can allow the manufacture of devices that combine component parts that have different properties (optical, mechanical, electrical, or thermal), realizing a product that has some combination of these properties. It can also be used as a way of combining distinct component parts, which require mutually exclusive manufacturing. Another advantage of this approach is to simplify the manufacture of, or improve the performance of, an end product, by removing the exposure of one component part to a manufacturing step that has a deleterious effect on the components performance in the end product.
This approach has been used to integrate silicon chips onto printed circuit boards. For example, a solder bumped flip chip process was introduced by International Business Machines Corporation in the early 1960's. This technology utilizes solder bumps deposited on wettable terminals on the chip and on the substrate. The solder-bumped chip is flipped and then aligned to the substrate and electrical connections are made by reflow of the solder. Similar flip chip techniques have been used to integrate gallium arsenide (GaAs) photodetector elements onto silicon microchips.
Organic electroluminescent devices have previously been manufactured that incorporate active matrix addressing schemes. Such devices have been demonstrated using polysilicon TFTs (thin film transistors) and CMOS (complementary metaloxide-semiconductor) crystalline silicon as the active addressing material. In all cases, this has involved the electroluminescent pixel structures being manufactured directly onto a substrate that houses the active matrix circuitry. This exposes the active matrix circuitry material to the processing steps required to build the pixel arrays, which can be disadvantageous.
For example, using the methods described in the prior art of the field, to manufacture a device with a CMOS active matrix addressing scheme involves a number of compromising processing steps. The fragile and sensitive CMOS substrate housing the active matrix driving circuitry is exposed to the potentially damaging environment, which is needed to manufacture the diode structures. The CMOS may also be exposed to (depending on whether small molecule, or polymer, emitting materials are used) a range of solvents, vacuum drying, and vacuum metallization processes, all of which can damage CMOS.
Conversely, by building the electroluminescent array onto the CMOS, the encapsulation of the device is compromised. This is because a transparent electrode structure must be processed on top of the light emitting material. This is routinely achieved by the deposition of a very thin layer (approximately 10 nm) of a certain metal, followed by the deposition of a thicker layer of transparent conducting material, typically indium tin oxide (ITO). The deposition of ITO exposes the underlying layers to both an oxygen rich and high temperature environment, both of which can reduce the lifetime of devices. This structure puts an oxygen rich compound (ITO), from. which oxygen can migrate, directly next to either oxygen sensitive, metals or organic materials. The device requires further encapsulation using an impermeable barrier such as glass.
Organic light emitting materials are sensitive to water, oxygen, certain organic solvents, and temperatures much above 100° C. Exposure to any of these can degrade the emissive properties, or cause catastrophic failure of a device. To build efficient diode structures it is also necessary to use other reactive materials in the electrode structures, such as thin layers of calcium, magnesium, or other metals that oxidize readily. To protect these sensitive materials it is necessary to encapsulate the devices, ideally with barrier layers that are impermeable to water and oxygen.
The non-planar topology of the active circuit chip, on which the electroluminescent pixels must be formed, causes difficulties in encapsulation. The non-planar topology can also cause variations in device performance across a pixel, and possibly cause catastrophic effects such as short circuits between the top and bottom electrode.
When active circuitry is used in optical applications, it is usually necessary to include light-blocking layers as part of the manufacturing process. This is to avoid the light affecting the electronics, by causing charge leakage. FIG. 1 shows, in schematic section, a conventional device in which light rays 1 are incident on top electrodes 2 as well as on light-blocking layers 3 between the top electrodes. An underlying dielectric 4 covers an active circuit 5 which is arranged on an active circuit substrate 6.